Document feeders for advancing original documents to a proper position for copying on the transparent imaging platen of an electrophotographic copier are well known in the art, being described, for example, in U.S. Pat. No. 3,829,083 issued to Shiina et al, U.S. Pat. No. 3,747,918 issued to Margulis et al, and U.S. Pat. No. 4,129,295 issued to Hori et al.
Recently there have been disclosed automatic document feeders of the "recirculating" type for successively feeding sheets from a stack of originals to the imaging platen of an electrophotographic copier. In one such feeder of this type, disclosed in U.S. Pat. No. 4,078,787, issued to Burlew et al, originals are separated from the bottom of a stack generally overlying the imaging platen, turned around and advanced to the imaging platen at which a single copy is made, and then again turned around and returned to the top of the stack.
While the so-called recirculating document feeders of this type can be used to produce collated sets of copies without any additional sorting step, they have several drawbacks. First, since it is necessary to make one full pass in which each original is recirculated for each set of copies desired, the additional time required to position each document on the imaging platen increases the total copying time.
Other problems arise in the separation of sheets from the stack. In the Burlew et al patent referred to, a vacuum cylinder is used as a sheet separator with its attendant bulk, complexity and requirement for the provision of a vacuum pump. Still other feeders such as disclosed in U.S. Pat. No. 4,231,562 issued to Hori employ a friction feed roller which cooperates with a reversely driven retarding roller for preventing the multiple feeding of sheets. In feeders of this type, the feed roller and elements rotating therewith are usually disposed a fixed distance from the retarding roller and associated elements, the distance being selected so as to permit the passage of only a single sheet between the respective rollers. Because of the fixed separation of the feed-roller and retarding-roller shafts, the performance of the feeder is dependent on the thickness of the sheets being handled, and sheet separation may become unreliable in the case of onionskin or other thin originals.
Finally, completely automatic feeders of the recirculating type have the disadvantage of being overspecialized. That is, the feeders operate most advantageously in the completely automatic mode and are not readily adaptable to operating semiautomatically. The place-marking element usually employed when operating in a recirculating mode also tends to increase the complexity and expense of the overall apparatus.